Hydrologic and Water Quality Performance of Four Bioretention Cells in Central North Carolina

Because bioretention areas have the ability to fulfill both landscape and water quality needs in a small area, they have received increased recognition as an integral part of Low Impact Development (LID); however, questions on design guidelines persist. Two paired, field-scale bioretention studies (Greensboro and Louisburg) have been conducted in North Carolina to study removal of phosphorus and nitrogen, hydraulic retention, the effectiveness of an induced saturated zone, and the impact of low P-Index fill media. Both bioretention pairs comprise nominally 5% of their respective watersheds and are planted with trees and shrubs. In Greensboro, one conventionally drained cell and one cell containing an induced saturated zone (previously termed anaerobic zone) of 0.45m were continuously monitored from June, 2002, to December, 2004. Lower outflow frequency was found for the induced saturated zone design. During outflow events, TP concentrations were significantly lower (P<0.01) than the conventional cell, although both designs increased outflow concentrations of nutrients. NO₃ concentrations were reduced by 77%; however, TKN and NH₄ concentrations were increased where the induced saturated zone was incorporated. In Louisburg, exfiltration, ET, and the significance of soil media for bioretention have been quantified by comparing outflow from two conventional cells constructed in winter of 2003 with non-agricultural soils. One cell was lined with 20 mil plastic; the other was unlined. During the summer of 2004, approximately 25% of inflow exfiltrated the cell, another 25% left via the cell's underdrains and 50% of the inflow was released as ET. Also in Louisburg, the non-agricultural fill soils resulted in average concentration reductions of 40–53% TP, and 25–60% TN for 10 storms.